Indirectly heated cathode (IHC) ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the chamber of the ion source. The cathode is disposed at one end of a chamber. A repeller is typically disposed on the end of the chamber opposite the cathode. The repeller may be biased so as to repel the electrons, directing them back toward the center of the chamber. In some embodiments, a magnetic field is used to further confine the electrons within the chamber.
In certain embodiments, electrodes are also disposed on one or more sides of the chamber. These electrodes may be positively or negatively biased so as to control the position of ions and electrons, so as to increase the ion density near the center of the chamber. An extraction aperture is disposed along another side, proximate the center of the chamber, through which the ions may be extracted.
One issue associated with IHC ion sources is that the cathode may have a limited lifetime. The cathode is subjected to bombardment from electrons on its back surface, and by positively charged ions on its front surface. This bombardment results in sputtering, which causes erosion of the cathode. In many embodiments, the life of the IHC ion source is dictated by the life of the cathode.
Therefore, an IHC ion source that can increase the life of the cathode may be beneficial. Further, it would be advantageous if this apparatus maintained the desired beam current throughout the life of the IHC ion source.